The siblings posed in their sports bras in photos shared to Instagram
Credit: Source: Instagram

Sister sweat session!

Kourtney Kardashian recruited Kylie Jenner to join her at the gym on Saturday, but getting the mini mogul to a yoga mat was apparently more of a challenge than their fitness routine.

Jenner posted a photo alongside her svelte older sibling over the weekend, championing the 36-year-old’s fit frame.

“First workout with @kourtneykardash this lady has been working hard,” she wrote.

Both women lounged on their sides in the image. The 18-year-old wore a gray and lime matching sports bra and leggings with white sneakers, while Kardashian donned a cut-out crop top with coordinating black leggings.

The ladies’ meet-up was apparently a long time in the making.

“I’ve been trying to get her to workout with me for six months,” Kardashian wrote on her own photo of their day. “Today was my lucky day.”

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Someone else has also noticed Kardashian’s hard work – Justin Bieber.

A source previously told PEOPLE that the 21-year-old singer “has had a huge ego boost from hooking up with Kourt.”

The source added of Bieber and Kardashian’s fling, “He brags about how hot she is. He is happy while it lasts.”